In general, the term “organic light emitting phenomenon” refers to a phenomenon in which electric energy is converted to light energy by means of an organic material. The organic light emitting device using the organic light emitting phenomenon has a structure usually comprising an anode, a cathode, and an organic material layer interposed therebetween. Herein, the organic material layer may be mostly formed in a multilayer structure comprising the layers consisting of different materials, for example, a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer, in order to improve efficiency and stability of the organic light emitting device. In the organic light emitting device having such a structure, when a voltage is applied between two electrodes, holes from the anode and electrons from a cathode are injected into the organic material layer, the holes and the electrons injected are combined together to form excitons. Further, when the excitons drop to a ground state, light is emitted. Such the organic light emitting device is known to have characteristics such as self-luminescence, high brightness, high efficiency, low drive voltage, wide viewing angle, high contrast and high-speed response.
The materials used for the organic material layer of the organic light emitting device can be classified into light emitting materials and charge-transporting materials, for example, a hole injecting material, a hole transporting material, an electron transporting material and an electron injecting material, according to their functions. The light emitting materials can be classified into high molecular weight materials and low molecular weight materials, according to their molecular weights. The light emitting materials can be classified into fluorescent materials derived from the singlet excited state and phosphorescent materials derived from the triplet excited state. The light emitting materials can be divided into blue, green and red light emitting materials, and yellow and orange light emitting materials required for giving more natural colors, according to the colors of the emitted light.
On the other hand, an efficiency of a device is lowered owing to maximum luminescence wavelength moved to a longer wavelength due to the interaction between the molecules, the deterioration of color purity and the reduction in light emitting efficiency when only one material is used for the light emitting material, and therefore a host/dopant system can be used as the light emitting material for the purpose of enhancing the color purity and the light emitting efficiency through energy transfer. It is based on the principle that if a small amount of a dopant having a smaller energy band gap than a host forming a light emitting layer is mixed in the light emitting layer, excitons which are generated in the light emitting layer are transported to the dopant, thus emitting a light with a high efficiency. Here, since the wavelength of the host is moved into the wavelength range of the dopant, a light having a desired wavelength can be obtained according to the kind of the dopant.
In order to allow the organic light emitting device to fully exhibit the above-mentioned excellent characteristics, a material constituting the organic material layer in the device, for example, a hole injecting material, a hole transporting material, a light emitting material, an electron transporting material and an electron injecting material should be essentially composed of stable and efficient materials. However, the development of stable and efficient organic material layer materials for the organic light emitting device has not yet been fully realized. Accordingly, the development of new materials is continuously desired.